Exothermic reactions and processes such as Fischer-Tropsch, and methanol formation from syngas, ethylene oxidation, maleic anhydride, phthalic anhydride, formaldehyde, acrylonitrile, acrylic acid, 1,2-dichloroethane, vinyl chloride, air compression, concentrated acid dilution, vapor condensation and others are strongly exothermic. Endothermic reactions and processes such as steam methane reforming and evaporation and others are strongly endothermic. Efficient heat transfer from/to reaction zone is required to improve the product selectivity, catalyst life and operational safety. A well-known reactor, the multi-tube reactor in a tube-and-shell configuration, has been used for highly exothermic or highly endothermic reactions. It is similar to a tube-and-shell heat exchanger, as shown in European patent No. 0,308,034. It consists of a number of thin tubes (usually less than 2 inches) in which catalyst particles are filled. These tubes are surrounded by cooling fluids, which pass through the shell side of the heat-exchanger-like reactor. Due to the high surface to volume ratio of the thin tubes, efficient heat exchange is able to be achieved. However, this design faced severe scale-up issues. At a larger scale, more thin tubes are required. The increasing part count makes the manufacture of such types of reactors very difficult and expensive, especially at large scales.
There were reactor designs that allow heating or cooling fluids passing through the tube side, and catalyst particles filled within space between shell and tubes. These designs are able to simply solve the scale-up issue of multi-tubular reactors, however, at the cost of heat exchanging efficiency. In this design family, different types of geometries of tubes have been used to improve heat exchanging efficiency, as shown in previous patents GB 2,204,055, U.S. Pat. Nos. 4,224,983, 5,080,872; different flow directions were also chosen for better heat exchanging performance.
These reactor designs, no matter how different they look, share one common structural characteristic: the reaction zone and heat exchanging zone are separated by tube walls. Therefore, these reactors are able to be classified as reactors with external heat exchanging.
Several new catalyst/reactor structure designs have been developed to improve the intra-bed heat transfer. The first approach is wash-coated monolithic catalyst structure, including metal honeycomb structure (U.S. Pat. Nos. 3,849,076; 4,101,287; 4,300,956; 6,869,578), metal monolith extrusion structure (U.S. Pat. Nos. 6,881,703; 7,608,344) and metal microchannel reactor (U.S. Pat. Nos. 7,084,180; 7,226,574; 7,294,734). This approach wash-coats a thin layer of catalyst on the internal wall of the monolithic structures. These structures made of thermal conductive materials (mainly metals or metal alloys) transfer heat fast from/to the reaction zone. Some of the catalyst structures have thick channel walls that allow heat exchanged on their external surfaces. For example, a catalyst structure (U.S. Pat. No. 7,608,344) is made by extrudating copper powders and then structure is formed by sintering or annealing in reducing environments. Copper forms a continuous phase that provides a very high thermal conductivity (e.g. 200 W/K-m), which equals to the product of bulk copper thermal conductivity and copper volumetric fraction (G. Groppi and E. Tronconi). The copper honeycomb structure transfers the heat from the wash-coated catalyst inside channel walls to the external honeycomb surface in an efficient way. Other monolithic or channel structures (U.S. Pat. Nos. 3,849,076; 4,101,287; 4,300,956; 6,869,578, 7,084,180; 7,226,574; 7,294,734) have thin channel walls and small channel sizes (usually several millimeters or less). In these cases, some channels have hot fluid and cold fluid passing through different channels next or cross to each other using the thin walls to separate the fluids and transfer heat. This design minimizes the heat transfer distance (resistance) and provides superior heat transfer performance at the cost of reactor complexity and reliability. In a word, the wash-coated monolithic structures significantly improve the heat transfer by reducing the heat transfer resistance and increasing the heat exchanging area. However, this wash-coated monolith approach, due to the nature of wash-coating, only allows a thin film of catalyst loaded inside the reactor channel. A typical catalyst volume loading is much less than 3 vol. %; some monolithic structures with small channel size (e.g. less than 1 mm) are able to reach a catalyst loading of 3-8 vol. %. Moreover, the mass transfer only take place by molecular diffusion in radial direction, which is much slower than the mass transfer in typical packed bed where bulk gas diffusion is dominant. The limited catalyst loading and low mass transfer rate results in slow reaction kinetics.
Another approach uses metal microfibrous media with catalyst entrapped for fast heat transfer. This type of media was first developed by Tatarchuk in 1992 (U.S. Pat. Nos. 5,080,963, 5,096,663). The media had good electrical conductivity and was developed as electrode materials for supercapacitors and fuel cells. Due to the similarity, the thermal conductivity of the materials should be predictable. Since 1994, this media has been modified for catalytic processes (U.S. Pat. Nos. 5,102,745, 5,304,330, 6,231,792, 7,501,012) and sorbent processes. In 2001, a novel reactor design with a folded microfibrous media sheet, which was parallel to the flow direction was proposed for fast heat transfer. This design also suffers from the slow radial molecular diffusion limit due to its parallel flow pattern. Moreover, the porous media only take negligible amount of reactor volume. Considering the low volume fraction of catalyst in this microfibrous media, the overall catalyst loading in the reactor will be extremely low. The folded structure has only several edge contacts with the reactor wall for heat transfer. This means the effective heat exchanging area is very limited. These drawbacks make the design much less competitive compared to the monolithic approach.